Many people, particularly those in middle age, are far-sighted, which means that they have little difficulty seeing objects at a distance, but have considerable difficulty seeing objects up close. These individuals have difficulty seeing print in a book dearly enough to read, so they need some sort of corrective lens or magnifier in order to read. When such an individual is outside in the sun and wearing sunglasses, a practical problem arises if that individual wishes to read a book. For example, he or she must purchase a pair of prescription sunglasses or endure the sun without the benefit of sunglasses. This practical problem is experienced by people needing prescription lenses for other purposes, as well. The purchase of a pair of prescription sunglasses is expensive and involves the further practical problem of carrying around an additional pair of prescription lenses (i.e., the prescription sunglasses).
One attempt to solve the problem of having to purchase and carry two pairs of prescription eyewear at the same time is to wear contact lenses. The contact lenses can be prescription lenses and thus the wearer need carry only a single pair of tinted eyewear for use as sunglasses. Many people have difficulty using contact lenses, however, and many of those who do use contact lenses find them to be irritating at times and to require substantial efforts to insert, remove, and clean.
Another approach has been to use a pair of prescription spectacles that are tinted in such a way that there is a minor tint when the ambient light is relatively low and a more severe tint as the ambient light becomes brighter. This approach is disadvantageous, however, because such spectacles are relatively expensive and the tint of the lenses may change at times when it is not desirable for it to change.
Yet another attempt to solve the problem of the need for two pairs of corrective eyewear is to use "clip-on" or "snap-in" tinted lenses in connection with a pair of clear prescription spectacles. Clip-on lenses are attached to a pair of eyeglasses by a clip, typically located at the center of the clip-on lenses. The clip attaches over the nosepiece of the eyeglasses. Sometimes the clip snaps over the sides of each of a pair of separate lenses formed to the shape of specific eyeglasses for which they are provided. In another variation, a tinted lens is snapped into a circumferential groove inboard of the clear lens. In another attempt to solve the problem, clear prescription lenses are hung, hinged or otherwise detachably mounted inboard of a pair of conventional sunglass lenses. Typically, some additional structure is added to the sunglass frames for holding a prescription lens or pair of lenses behind the sunglass lenses; however in one species, the frames themselves are modified with threads to receive a "screwed-in" prescription lens for each lens of the sunglasses. Clip-on lenses can be disadvantageous, however, because they result in less than desirable optics, typically through an inability to control the spacing and angular relationship between the sunglass lens and its clear lens with any precision or degree of stability, and because they are cosmetically unattractive.
Heretofore, some attempt has even been made, to meet the needs of the military, to combine the qualities of a toric lensed sunglass with prescription lenses, by actually inserting prescription lenses into voids cut out of the sunglass lenses themselves.
Recently, 3M company has provided on die cut sheets small `half-lens` shaped magnifiers cut from relatively flat vinyl stock and which employ a molded in Fresnel type grating for an optical magnification effect. These 3M lenses also suffer from optical aberrations and poor performance, especially when adhered to the panes of a pair of spectacles. In addition, the magnifying power of the Fresnel material does not adapt well to bending and it is also not readily adaptable to prescription configurations.
There is a need for a simple, inexpensive corrective lens that may be used in connection with a pair of fashion or performance sunglasses to provide a simple magnified or corrective pair of sunglasses that can be used comfortably and effectively for sustained periods of time. Thus, there is a need to adapt non-corrective sunglasses for use as corrective sunglasses without the above noted disadvantages. The present invention provides these and other advantages.